Micromachined structures are frequently used as sensors or actuators and micromachined accelerometers, in particular, are widely used to detect and measure acceleration, tilt, or vibration for many applications. Among micromachined accelerometers, the differential capacitor type is typically used.
A differential-capacitor based accelerometer typically includes a micromachined sensor and its excitation and readout electronics. The micromachined sensor typically includes several primary micromachined elements; a movable mass, support springs, and capacitor plates for sensing the displacement of the movable mass. Often, additional actuator plates are provided to implement a self-test function. FIG. 1 shows an exemplary prior art differential-capacitor acceleration sensor 300. The movable mass 302, supported by springs 314, 316, 318, 320, is positioned midway between two plates so that one capacitor is formed by a first plate and the mass and a second (and equal) capacitor is formed by a second plate and the mass. Together these form the differential capacitor for sensing the displacement of the movable mass. To maximize the accelerometer capacitance, the mass may contain numerous fingers 350 that are interleaved between fingers from the two plates 348, 352. Likewise is the construction of the actuator plates 360, 362. These plates are used to generate electrostatic forces via a voltage applied, for example, between the fingers of the actuator plate 360 and the fingers 358 of the mass 302. The electrostatic force deflects the mass 302 and produces an output response that can be used for self-testing or other test purposes, e.g., measuring the resonant frequency of the movable spring-mass system.
The sensitivity of a micromachined accelerometer is determined by a variety of factors, including spring constant, mass of certain elements (e.g., proof mass), sense and parasitic capacitances, and electronic gain. As a result of the small dimensions involved, (on the order of micrometers), the sensitivity of a micromachined accelerometer may vary significantly due to manufacturing variations which alter dimensions of micromachined structures within the accelerometer. Among these, spring constant and sense capacitance see the most variations. Accordingly, some post-manufacturing calibration is typically required. Effective calibration requires an accurate determination of sensitivity followed by, if necessary, electronic gain adjustments in the form of EPROM programming, laser trimming, or electrical fusing of circuit elements, to bring sensitivity to target.
Commonly, the sensitivity of these accelerometers is measured with a device that shakes the accelerometer either at the wafer or the packaged device stage of manufacturing. Alternatively, the orientation of the accelerometer can be changed precisely with respect to the gravitational field, for example, with a rotator and the output response can then be calibrated to gravity. Testing involving precise mechanical excitations is very costly, especially if it is required to be performed over temperature. It is the main obstacle barring micromachined accelerometers from realizing the low cost of standard IC testing. A method of accurately measuring the sensitivity of these accelerometers without requiring equipment for mechanical manipulation of these devices (e.g., shaking) could significantly reduce manufacturing costs.